Problem: If $4a + 8b + 9c = 3$ and $7x + 5y = 1$, what is $-81c - 72b - 15y - 36a - 21x$ ?
Answer: $= -36a - 72b - 81c - 21x - 15y$ $= (-9) \cdot (4a + 8b + 9c) + (-3) \cdot (7x + 5y)$ $= (-9) \cdot (3) + (-3) \cdot (1)$ $= -27 - 3$ $= -30$